Conventional videotape recorders have an automatic gain control circuit which measures the level of a pulse (called xe2x80x9csync pulsexe2x80x9d) in a video signal. The circuit uses the measured level to develop a gain correction signal that is used to keep the video level applied to an FM modulator in the videotape recording system (VCR) at a fixed, predetermined value. It is conventional to prevent unauthorized copying of a videotape, by use of the fact that such a circuit of a videotape recorder is sensitive to pulses added to a normal video signal to prevent copying as described in, for example U.S. Pat. No. 4,631,603 granted to Ryan (i.e., Ryan ""603, that is incorporated by reference herein in its entirety).
U.S. Pat. No. 5,583,936 granted to Wonfor et al. (i.e., Wonfor ""946, also incorporated by reference herein in its entirety) describes a number of enhancements (hereinafter xe2x80x9cretrace enhancersxe2x80x9d) to a video anti-copying process of the type described above or to anti-copy processes that cause attenuation (as described in e.g. U.S. Pat. Nos. 5,130,810 and 4,918,098 granted to Ryan (Ryan ""810 and ""098) and 4,163,253 granted to Morio et al. (Morio ""253). Specifically, Wonfor ""936 describes e.g. introduction into the overscan portion of the television picture, just prior to the horizontal or vertical sync signals but in active video, a negative going waveform that appears to the television receiver (TV set) or videotape recorder (VCR) to be a sync signal, thereby causing an early horizontal or vertical retrace.
One version provides a waveform (called horizontal retrace enhancer) in the right overscan portion of the picture, that results in a checker pattern of alternating gray and black areas (illustrated in FIG. 1A as ES0-ES6). This causes the TV set on which the illegal copy is played to horizontally retrace earlier than normal in selected lines, with a consequential horizontal shift of the picture information on those lines. This substantially degrades picture playability. In another version, another waveform (called vertical retrace enhancer) introduces a gray pattern (illustrated in FIGS. 1B and 1C as GP0 -GP2 and GP3-GP5 respectively) at the bottom overscan portion of the picture, which causes vertical picture instability (also called xe2x80x9cjitterxe2x80x9d). It is noted that the features in the waveforms of FIGS. 1A-1C designated as ES0-ES6, GP0-GP2 and GP3-GP5, are illustrated in dashed lines to indicate that the video levels thereof are variable.
Circuitry 10 for inserting the above-described horizontal and vertical retrace enhancers is shown in block form in FIG. 1D. The main video signal path includes an input clamp amplifier A1 (or DC restored amplifier), a sync pulse narrowing circuit 13, a mixing point 15 at which the waveform components of the checker pattern and the vertical modification (jitter inducing) waveforms are added, and an output line driver amplifier A2. Amplifier A1 ensures that the video signal at blanking is at a known pre-determined DC level before adding any additional waveform components to that video signal. The resulting clamped video signal is applied to mixing point 15 with a source impedance Ro, typically greater than 1000 ohms. In this case also the video input signal into circuit 10 of FIG. 1D may have the last 9 lines of each field blanked to a reference level. U.S. Pat. No. 4,695,901(i.e., Ryan ""901, incorporated by reference herein in its entirety) shows a switching circuit for blanking.
A process control and signal generation path includes a sync separator 11, a control circuit 12, circuits VR1-VR4 to generate the required signal voltages which will be added to the main video signal, and a switch selection system 14-1 through 14-4 which applies the required signal voltages to the mixing point 15 under the control of the control circuit 12. That is, the retrace enhancers to be injected are applied to the mixing point 15 with a source impedance typically less than 50 ohms. The input video is DC restored by the input video clamp amplifier A1.
When it is required to modify the input video signal, for example with a checker component, the appropriate signal is selected and applied to the mixing point 15 at the low source impedance, which overrides the input video signal from amplifier A1 and effectively replaces the input video signal with the required signal. When the input signal is to remain unchanged, the switch elements 14-1 to 14-4 are all in the open state, with the result that the video signal passes unchanged to the output line driver amplifier A2. The resulting video signal at the mixing point 15 is applied to line driver amplifier A2 to provide standard output signal level and output impedance. Sync separator 11 provides composite sync pulses and a frame identification signal required by the process control circuit 12. The process control circuit 12 generates control signals to turn on the signal selection switches 14-1 to 14-4 at the precise time and for the required duration that the various signals, that is, the horizontal or vertical retrace enhancers, are to replace the input video signal.
Various signals which replace the input video signal consist preferably of a high or low steady state DC signal level. For example the checker signal xe2x80x9chigh levelxe2x80x9d is a mid-gray level, typically of about 30% of peak white, whereas the checker signal xe2x80x9clow levelxe2x80x9d is black level or blanking level. These various level signals are generated from potentiometers VR1, VR2, VR3, VR4 (or alternatively from voltage divider resistors for fixed preset signal levels) connected across appropriate supply voltage lines. These signals are applied to the corresponding selection switch elements 14-1, 14-2, 14-3, 14-4 via unity gain operational amplifiers to ensure the required low output impedance into the mixing point 15.
Control circuit 12 generates the appropriate switch selection control pulses for addition of the horizontal and vertical retrace enhancers (see FIGS. 3A-3C). Checker pulses (horizontal retrace enhancers) are applied only to selected lines. One example starts the checker pattern at the tenth line containing picture information, that is, after the end of vertical blanking and ends it ten lines before the last line containing picture information, that is, 10 lines before the start of the succeeding vertical blanking interval. Similarly, the vertical jitter signals (vertical retrace enhancers) are applied only to selected lines, for example the last nine lines prior to the vertical blanking interval. Hence, both the checker pattern and vertical jitter signals require control signals with both horizontal and vertical rate components.
The video input signal also is buffered by the amplifier A3 and is coupled to the sync separator 11 which provides the composite sync pulses and frame identification (e.g., square wave) signals of previous mention. The composite sync pulses are applied to a phase-locked loop (PLL) located in the control circuit 12. The phase control of the PLL using a potentiometer is adjusted so that the horizontal rate output pulse starts at the required start point of the checker, typically two microseconds before the start of horizontal blanking. The output signal of the PLL is used to derive the horizontal rate component of both the checker and vertical modification signals.
A burst gate output signal from the sync separator 11 is inverted by an inverter U5 which provides a clamping pulse for clamp amplifier A1. It is to be understood that the circuit 10 (FIG. 1D) can be coupled to receive an input signal from a circuit that implements an attenuation-based anti-copy process.
Wonfor ""936 describes a sync pulse narrowing circuit and method that is used by itself or in cascade (as shown in Wonfor""s FIG. 6a block 96; see also FIGS. 13A, 13B, 14A and 14B) with any other signal modification techniques. The method narrows the video signal sync (synchronization) pulses, mainly the horizontal sync pulses, so that when an illegal copy is made, the attenuated video with reduced sync pulse width, that is, duration, causes a playability problem (also called xe2x80x9cplayability problemxe2x80x9d) when viewed on a TV set. This is because most TV set sync separators incorporate sync tip DC restoration. Because these TV set sync separators are typically driven by medium impedances, the sync pulses are partly clipped off. By narrowing the sync pulses, the sync pulses are even further clipped off. When an unauthorized copy is made of the video signal, especially with the above-described checker and/or end of field modification signal, the copy has a reduced amplitude video with reduced sync pulse width. As a result, the TV sync separator sees a severe loss in sync due to its own clipping from the narrowed sync width and the attenuation of the video itself. Thus, the TV set""s sync separator does not xe2x80x9cextractxe2x80x9d sync properly and this causes the TV picture to be even less viewable, because the horizontal and/or vertical modification effects are more intense.
In accordance with the invention, an apparatus and method change at least one attribute of copy protection (for example, checker signals, sync narrowing, vertical jitter pulses, and/or sync amplitude changes) that is present in or is to be added to a video signal, to control the display of contents carried by the video signal, for example, the to-be-displayed audio or video contents. Changing one or more pulses, such as any sync pulse or the retrace enhancer as described above, in a manner that is not predetermined as described herein, enhances prior art methods. Such change is also called xe2x80x9cdynamic changexe2x80x9d. For example, such dynamic alteration in accordance with the invention increases the effectiveness of copy protection during display of dark or less bright scenes so as to make more use than does the prior art, of the retrace enhancers and/or narrowed sync pulses, which may be otherwise not effective in dark or less bright scenes. The amount of dynamic change is selected to ensure that an unattenuated version of the video signal carrying the changed pulse(s) can be viewed in the normal manner, but an attenuated version of the video signal (e.g., a copy) causes playability problems. It should be noted that the modifier (i.e., dynamic) enhancement signals of the present invention, preferably are combined with a basic copy protection signal, for example, a modified video signal that causes a change in the automatic gain control (AGC) level in a VCR.
A dynamic change in copy protection that is made in one embodiment is related to a scene""s content that is carried by the video signal. In one implementation, the attribute that is changed is the low level of a retrace enhancer, thereby making the retrace enhancer xe2x80x9cscene content or brightness sensitive.xe2x80x9d In a first example of this implementation, a low level of a horizontal retrace enhancer is changed, such as a checker pattern that is added at the end of a line (EOL) as described in Wonfor ""936, and the change is related to a property of the to-be-displayed scene, such as brightness or scene content (that is, average picture level, APL). In addition to or instead of changing a pulse related to an end of line (EOL) as described above, one or more attributes of a pulse related to an end of field (EOF) can also be dynamically changed depending on scene content carried by the video signal. Therefore, in a second example, the low level of a vertical retrace enhancer that is added to one or more groups of lines in the active video but in overscan region of a video signal, for example, an intermittently added peak white signal or mid gray signal as described in Wonfor ""936, is changed. In the second example, depending on the variant, a color burst signal can be carried unmodified (i.e. at the low level), or can be carried at the high level by the vertical retrace enhancer being added.